In order to eliminate potential ambiguities — after having simplified (REF ), (REF )
using 4-dimensional Lorentz and Dirac algebra as much as possible —, we should agree on one definite fixed form of
(REF ), (REF ), solely in terms of a string of Dirac \(\gamma \)  matrices with
fixed product ordering, the Levi-Civita tensor, and external momenta.
We may call these their canonical forms in 4 dimensions.
This allows an unambiguous interpretationThis is up to a potential subtlety
related to the contraction order among multiple Levi-Civita tensors [1]},
as will be commented on in section REF .
of the expression in D dimensions where it will be manipulated according to the \(D\) -dimensional algebra
after being inserted back into eq. (REF ).
